The present invention relates to viscous fluid couplings, and more particularly to such couplings which are used to drive vehicle radiator cooling fans, wherein it is desired to control the engagement or disengagement of the viscous fluid coupling in an open loop manner.
Viscous fluid couplings have been in commercial use for many years to drive vehicle radiator cooling fans, especially on vehicles such as automobiles and light trucks. The use of such viscous couplings has been widespread because such couplings can operate in either an engaged condition, or a disengaged condition, depending upon a sensed temperature condition, indicating the need, or lack of need, respectively, for cooling of the radiator. Most of the couplings (viscous fan drives) which have been in commercial use include some form of temperature-sensing bimetalic element which senses ambient air temperature adjacent the fan drive, and controls valving within the fan drive in response to the sensed temperature, to achieve either the engaged or the disengaged condition, as is appropriate.
In certain vehicle applications, it has become desirable to sense directly the temperature of the liquid coolant entering the radiator, and to control the valving within the viscous fan drive in response to the coolant temperature, whereby the responsiveness of the fan drive is improved, when compared to the conventional fan drive which senses only the ambient air temperature.
Typically, the sensed temperature (for example, the "top tank" temperature at the radiator) has been converted into an electrical signal, and used to control the movement of the valving by some sort of an electrical or electromechanical means. An example of such a viscous fluid coupling, is illustrated and described in U.S. Pat. No. 5,152,383, assigned to the assignee of the present invention, and incorporated herein by reference.
In electromagnetically controlled viscous fluid couplings, it has been considered necessary to sense actual fan speed, and generate a corresponding feedback signal which is then transmitted to the vehicle microprocessor. This feedback signal is used to modify the main command signal to the electromagnetic means which controls the valving of the coupling. In other words, such viscous coupling devices have been controlled by a control system of the "closed loop" type as that term is generally understood by those skilled in the control art. Such closed loop control can provide excellent performance, in terms of achieving engagement or disengagement of the coupling, as is appropriate. However, the need to sense actual fan speed and provide a feedback signal, etc., adds substantially to the cost and complexity of the coupling and its control system.
Accordingly, it is an object of the present invention to provide an improved viscous coupling, device in which output coupling speed (fan speed) can be controlled accurately, relative to an electrical input signal, without the cost and complexity of a closed loop control system, i.e., in which the output coupling speed can be controlled by controlling the coupling in an "open loop" manner.
One of the reasons for the difficulty of achieving accurate open loop control of a viscous coupling has been the lack of predictability of the level of fluid fill in the operating chamber for a given level of input to the valving of the coupling. One reason for this lack of predictability in a conventional coupling device is that the fluid fill level represents an attempted balance between the fill rate at the inlet port and the discharge rate at the pump out port. As is known to those skilled in the art, the fill rate is typically a function of the output speed of the coupling device, because in the conventional coupling device, the fluid reservoir is disposed within the output member, and rotates at output speed.
Therefore, it is another object of the present invention to provide an improved fluid coupling device having improved valving whereby there is a more predictable relationship between fill level in the operating chamber (output speed) and the input signal to the valving than was possible in prior art couplings and control systems.
It is a related object of the invention to improve the predictability of the output speed vs. input signal relationship by making the pressure in the fluid reservoir independent of output speed, i.e., by providing a coupling device of the reservoir-in-clutch type.
It is another related object of the invention to be able to maintain certain fan speeds, and especially certain intermediate fan speeds accurately, as a function of input signal, by the design (geometry) of the fluid reservoir, somewhat independent of tolerances of the various parts.
For a number of years it has been known in the viscous coupling art to provide valving having "modulation" capability. Such capability is illustrated and described in U.S. Pat. No. 3,227,254, assigned to the assignee of the present invention and incorporated herein by reference. In a modulating fluid coupling device, rather than the fill port opening suddenly, whereby there is a relatively high gain rate in the output speed versus the valve position, the opening of the valving occurs over a wider range of inputs, for a lower gain rate of output speed versus input to the valving. Unfortunately, it has been observed in some modulating type fluid coupling devices that the desire to provide the modulation capability has interfered with or hindered the achievement of certain other important performance characteristics, such as the required peak speed.
Accordingly, it is an additional object of the present invention to provide an improved fluid coupling device of the modulating type which is capable of achieving good modulation performance while still being capable of achieving other required performance characteristics.